Using NUnit testing with WinRT application

I am having trouble getting NUnit to work with my Windows 8.1 application. I have been working on this application for a few months and am hoping to start some unit testing with NUnit.

I have added a new project to my solution and added the NuGet package for NUnit to the new project. Now I am trying to add a reference to my main project, which is in the same solution. However, I keep getting "Unable to add reference to project" errors every time.

After googling around a bit, I am not sure whether or not this is possible. The unit test project is just a blank project, I haven't written any tests yet. I am simply trying to add a reference to the main project so that I will be able to reference it in the tests. Has anyone had any luck getting NUnit to work with WinRT apps?